Humberto O. Stumpf

Magnetic properties of a novel molecule-based ferrimagnet exhibiting multiple magnetic pole reversal

The magnetic properties of a new molecule-based ferrimagnet containing three different spin sources have been studied. Below the magnetic ordering temperature, the magnetic poles reverse once in a small external field as the temperature varies. At a higher field, they reverse twice at temperatures that depend on the measurement process. A double thermal hysteresis is observed in a constant field. These experimental results can be rationalized by a combination of the molecular-field theory (MFT) and magnetic anisotropy.

Molecular ferromagnetism : orbital approach and recent achievements

Abstract This paper is divided into two parts. First the various orbital patterns favoring a ferromagnetic interaction between molecular units are explored. Two types of mechanisms are distinguished according to whether they require or not an electron transfer between the molecular units. As far as the mechanisms requiring no electron transfer are concerned, a further distinction is made between spin delocalization and spin polarization. Each concept is illustrated by at least one example. The second part deals with a new family of molecular-based magnets of formula Mn2[Cu(opba)]3R2nL where opba stands for orthophenylenebis(oxamato), R for a monovalent cation and L for a solvent molecule. The compound Mn2[Cu(opba)]3(NBu4)2.6DMSO exhibits a spontaneous magnetization below 15 K.

A two-dimensional oxamate- and oxalate-bridged Cu(II)Mn(II) motif: crystal structure and magnetic properties of (Bu4N)2[Mn2{Cu(opba)}2ox].

A new compound of formula (Bu4N)2[Mn2{Cu(opba)}2ox] (1) [Bu4N(+) = tetra-n-butylammonium cation, H4opba = 1,2-phenylenebis(oxamic acid), and H2ox = oxalic acid] has been synthesized and magneto-structurally investigated. The reaction of manganese(II) acetate, [Cu(opba)](2-), and ox(2-) in dimethyl sulfoxide yielded single crystals of 1. The structure of 1 consists of heterobimetallic oxamato-bridged Cu(II)Mn(II) chains which are connected through bis-bidentate oxalate coordinated to the manganese(II) ions to afford anionic heterobimetallic layers of 6(3)-hcb net topology. The layers are interleaved by n-Bu4N(+) counterions. Each copper(II) ion in 1 is four-coordinate in a square planar environment defined by two amidate-nitrogen and two carboxylate-oxygen atoms from the two oxamate groups of the obpa ligand. The manganese(II) ion is six-coordinate in a somewhat distorted octahedral surrounding that is built by two oxalate-oxygen and four carbonyl-oxygen atoms from two [Cu(opba)](2-) units. The magnetic properties of 1 in the temperature range 1.9-300 K correspond to those expected for the coexistence of intralayer antiferromagnetic interactions of the type copper(II)-manganese(II) across oxamato and manganese(II)-manganese(II) through oxalato bridges plus a weak spin canting in the very low temperature domain. Simulation of the magnetic data through quantum Monte Carlo methodology reveals the magnitude of the intralayer magnetic interactions [J(CuMn) = -32.5(3) cm(-1), and J(MnMn) = -2.7(3) cm(-1)], their values being within the range of those previously observed in lower nuclearity systems.

Cluster glass-like behavior in a 2D bimetallic molecule-based magnet

The compound of formula Na2[Co2{Cu(opba)}3]/2DMSO/6H2O has been synthesized where opba stands for ortho phenylenebis(oxamato) and DMSO is dimethylsulfoxide. Magnetic properties have been investigated in the 2/300 K temperature range. The compound presents a large coercive field, around 3.5 kOe at 4.2 K, and the A.C. susceptibility measurements has revealed a cluster glass-like behavior. # 2003 Elsevier Science Ltd. All rights reserved.

New properties of the [3-MeRad]2[Mn2{Cu(opba)}3] system [3-MeRad+=radical cation, opba=ortho-phenylenebis(oxamato)]: a molecule-based magnet with two-dimensional behavior

Abstract Three compounds have been synthesized, the formulae of which are [3-MeRad] 2 [Cu(opba)]·4H 2 O ( 1 ), [3-MeRad] 2 [Mn 2 {Cu(opba)} 3 ]·6DMSO·8H 2 O ( 2 ), and [3-MeRad] 2 [Mn 2 {Cu(opba)} 3 ]·3DMSO·2.5H 2 O ( 3 ), where opba is ortho -phenylenebis(oxamato) and 3-MeRad + is 2-(3- N -methyl-pyridinium)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide. Their magnetic properties have been investigated in the 1.8–300 K temperature range. By reaction of 1 and MnCl 2 ·3H 2 O in DMSO, small crystals of 2 were obtained. Single crystal X-ray analysis was employed to determine the lattice parameters for 2 : a =15.9(2); b =17.8(2); c =18.0(3) A; α = γ =90(1); β =114(1)°. The temperature dependence of the magnetic susceptibility in the form of the χ M T versus T plot reveals a typical ferrimagnetic behavior for 2 , although the low field FC and ZFC curves show an antiferromagnetic-like interaction below 13 K. Above 200 Oe this interaction disappears as in weak metamagnets. With a thermal-vacuum treatment applied to 2 , solvent molecules are removed affording 3 which presents long-range order around 15 K. The results indicate that the bimetallic compounds present a structure consisting of two-dimensional honeycomb-like networks without interlocking of the M 6 Cu 6 hexagons.

Photoluminescent and Slow Magnetic Relaxation Studies on Lanthanide(III)-2,5-pyrazinedicarboxylate Frameworks

In the series described in this work, the hydrothermal synthesis led to oxidation of the 5-methyl-pyrazinecarboxylate anion to the 2,5-pyrazinedicarboxylate dianion (2,5-pzdc) allowing the preparation of three-dimensional (3D) lanthanide(III) organic frameworks of formula {[Ln2(2,5-pzdc)3(H2O)4]·6H2O}n [Ln = Ce (1), Pr (2), Nd (3), and Eu (4)] and {[Er2(2,5-pzdc)3(H2O)4]·5H2O}n (5). Single-crystal X-ray diffraction on 1-5 reveals that they crystallize in the triclinic system, P1̅ space group with the series 1-4 being isostructural. The crystal structure of the five compounds are 3D with the lanthanide(III) ions linked through 2,5-pzdc2- dianions acting as two- and fourfold connectors, building a binodal 4,4-connected (4·648)(426282)-mog network. The photophysical properties of the Nd(III) (3) and Eu(III) (4) complexes exhibit sensitized photoluminescence in the near-infrared and visible regions, respectively. The photoluminescence intensity and lifetime of 4 were very sensitive due to the luminescence quenching of the 5D0 level by O-H oscillators of four water molecules in the first coordination sphere leading to a quantum efficiency of 11%. Variable-temperature magnetic susceptibility measurements for 1-5 reveal behaviors as expected for the ground terms of the magnetically isolated rare-earth ions [2F5/2, 2H4, 4I9/2, 7F0, and 4I15/2 for Ce(III), Pr(III), Nd(III), Eu(III), and Er(III), respectively] with MJ = 0 (2 and 4) and ±1/2 (1, 3, and 5). Q-band electron paramagnetic resonance measurements at low temperature corroborate these facts. Frequency-dependent alternating-current magnetic susceptibility signals under external direct-current fields in the range of 100-2500 G were observed for the Kramers ions of 1, 3, and 5, indicating slow magnetic relaxation (single-ion magnet) behavior. In these compounds, τ-1 decreases with decreasing temperature at any magnetic field, but no Arrhenius law can simulate such a dependence in all the temperature range. This dependence can be reproduced by the contributions of direct and Raman processes, the Raman exponent (n) reaching the expected value (n = 9) for a Kramers system.

From antiferromagnetic to ferromagnetic exchange in a family of oxime-based MnIII dimers: a magneto-structural study

The reaction of Mn(ClO4)2·6H2O, a derivatised phenolic oxime (R-saoH2) and the ligand tris(2-pyridylmethyl)amine (tpa) in a basic alcoholic solution leads to the formation of a family of cluster compounds of general formula [Mn(III)2O(R-sao)(tpa)2](ClO4)2 (1, R = H; 2, R = Me; 3, R = Et; 4, R = Ph). The structure is that of a simple, albeit asymmetric, dimer of two Mn(III) ions bridged through one μ-O(2-) ion and the -N-O- moiety of the phenolic oxime. Magnetometry reveals that the exchange interaction between the two Mn(III) ions in complexes 1, 3 and 4 is antiferromagnetic, but that for complex 2 is ferromagnetic. A theoretically developed magneto-structural correlation reveals that the dominant structural parameter influencing the sign and magnitude of the pairwise interaction is the dihedral Mn-O-N-Mn (torsion) angle. A linear correlation is found, with the magnitude of J varying significantly as the dihedral angle is altered. As the torsion angle increases the AF exchange decreases, matching the experimentally determined data. DFT calculations reveal that the dyz|π*|dyz interaction decreases as the dihedral angle increases leading to ferromagnetic coupling at larger angles.

NMR AND SEMIEMPIRICAL CONFORMATIONAL ANALYSIS OF THE 2-ARYL-1,3-DIHYDROXY-4,4,5,5-TETRAMETHYLIMIDAZOLIDINES

Abstract 2-Aryl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidines (1) were synthesized and their 1 H and 13 C NMR spectra recorded. Quantum mechanical semiempirical calculations were also performed, for a better understanding of the signals recorded in the NMR spectra of imidazolidines. The conformation of the imidazolidine ring was initially studied for the 2-methyl-1,3-dihydroxy-4,4,5,5-tetramethylimidazolidine (2) , used as a model molecule. The results of the calculations obtained for structure 2 showed that the methyl groups are located in axial and equatorial positions. At these positions, the methyl groups are affected by the magnetic anisotropic effects of carbon-nitrogen and carbon-carbon bonds of the imidazolinyl ring with different intensities. Semiempirical calculations for structure 2 , suggested that the effect of the γ-oxygen on the carbon atoms of methyl groups (γ-effect) might lead to an alteration of the electronic charge density and consequently to a change in the diamagnetic shielding on these groups. These data were used for the analysis of the NMR spectra of compound 1 . The diamagnetic shielding effects, estimated from the calculated electronic charge densities on the carbon atoms of methyl groups, are in agreement with the signals observed in the NMR spectra of compound 1 . By combining the contribution of the anisotropic and γ-effects, it appears that the axial methyl groups are located relatively closer to the γ-oxygens in compounds 1 .

Magnetic pole reversal and thermal hysteresis in molecule-based magnets with a fully interlocked structure

Abstract This paper concerns the chemistry and the physic of two compounds with a fully interlocked structure. The general formula is [Etrad2M2{Cu(opba)}3]·S, with M=Mg2+, Ni2+, where Etrad stands for an organic radical cation, opba stands for ortho-phenylenebis(oxamato) and S for solvent molecules (DMSO and/or H2O). The structure consists of two interpenetrating graphite-like networks with edge sharing hexagons. The magnetic properties have been investigated in detail. The Mg2+ derivative behaves as a paramagnet with weak ferromagnetic interactions between Cu2+ and Etrad+ ions whereas the Ni2+ derivative is a molecule-based magnet below Tc=28 K. The magnetization changes its sign below Tc at the so-called compensation temperature Tcomp under a small external field. For a higher field the magnetic poles may reverse two times at temperatures which depend on the measurement process (warming or cooling mode) and/or field history. A double thermal hysteresis is observed in a constant field in cycling the temperature between 35 and 2 K. Furthermore, the strength of the cooling magnetic field is strongly memorized because the magnetic poles orientation at any field or any temperature is directly related to the field history of the sample. We show that these original results can be understood on the basis of the combination of the molecular field theory and the magnetic anisotropy.

Synthesis and Properties of New 2-[2-(N-alkyl)-pyridinium]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide Iodide Radicals

Abstract2-(2-pyridyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide radical 2 and the 2-[2-(N-alkyl)-pyrid-inium] derivatives (3a and 3b) were synthesized and characterized by elemental analysis, melting points, and spectroscopy. The density functional theory (DFT) was used in order to obtain the structures and electronic properties of the new nitronyl nitroxide radicals. Modified reaction procedures of the intermediates are described with better yield and purity of the final products. The magnetic properties of the compounds 2, 3a, and 3b are predicted for the first time using a simple model of charge transference, in the framework of the molecular orbital calculations. The obtained results show that the substituent at the central carbon atom of the imidazoline is important to determine the spin distribution and consequently the nature of the magnetic interaction.